Throw it in a microwave. That is the way i sanitize my bamboo cutting boards when there are tiny traces of meat inside after tenderizing. If that bacteria have water molecules in them they will be dead. And if they don't - you may be sick but you will get nobel prize in biology anyway.
you can also dip stuff in acetone to smooth it and perhaps enamel it to really seal it up. I imagine there's a bunch of stuff you could do to improve the odds.
However, treating a 3d printed spoon like any old metal spoon adds risk.
Is the heating really even enough for that to work? Just from my experience heating viscous soups and the like, I'd expect there to be some spots that never get all that hot, leaving lots of surviving bacteria.
